For vitamin extractions, formulation samples (25 mg) were
added to tubes containing iso-propanol and the internal standard
(15LmL−1) and vortexed for 3 min. After centrifugation of
the suspension the filtered supernatants were used for HPLC
injection. To quantify retinyl palmitate and tocopheryl acetate
in the formulations, standard solutions were prepared daily and
analyzed by HPLC in parallel to samples. Peak-area ratios were
used for calculations following the internal standard method.